(1) Field of the Invention
The present invention relates particularly to DC offset correction in a mobile communication system.
(2) Description of the Related Art
In a mobile communication system, signals are transmitted from mobile stations to a base station. A digital signal is prepared for transmission by the mobile station by subjecting it to a modulation technique and using the resulting signal to modulate a carrier wave at a certain frequency. In transmission of the signal from a mobile station to the base station, it may be subject to a number of different effects, depending on the environment through which the signal passes. That environment can vary considerably, depending, amongst other things, on the distance between the mobile station and the base station, and the interference caused by buildings and other structures in the area. It is quite common for a signal received from a mobile station at the base station to comprise a number of different multi-path effects and also to be subject to noise. Processing techniques at the receiver in the base station are known to resolve the effects of the environment through which the signal passes (communication channel) and also to take into account the effects of noise. The first step at the receiver is to sample the incoming signal to take a number of digital samples from the incoming analogue signal, normally sampled at the expected bit rate of the transmitted signal. This can give rise to a DC offset component which, if not removed, could corrupt the received signal samples such that the subsequent processing would be affected. In particular, the digital signal samples may be processed by an equaliser to compensate for the effects of the channel, and the known equalisers do not assume that a DC offset will be present. There are other sources that may introduce a DC offset and the magnitude of the DC offset may vary. It is an aim of the present invention to obtain a reliable estimate of the offset magnitude that needs to be removed from the signal, without degrading the performance too much in the case that in fact no DC offset has been introduced.
In current base stations, a possible DC component is removed from the received signal by digital signal processing means. This is performed by removing the mean signal level from the real and imaginary component separately. Thus, the received signal y is considered to comprise both a real component and an imaginary component which are handled separately. A mean value (Ey) can be calculated over a whole burst to improve reliability. However, the transmitted data itself can cause the average value to have a false mean value, even in the case where there is no actual DC offset. This clearly degrades the performance of the subsequent digital signal processing.
According to one aspect of the present invention there is provided a method of simultaneously determining a DC offset (adc) and a channel impulse response (h) for a signal received from a first station by a second station via a communication channel in a mobile communication system, the signal comprising digital data and a set of training sequence bits modulated prior to transmission, the method comprising: generating a set of reference signal samples representing the training sequence bits and a set of rotation elements depending on the modulation applied to the digital data prior to transmission; receiving and sampling the signal to produce a plurality of received signal samples from the training sequence portion of the signal, the received signal samples possibly including a DC offset; and manipulating the received signal samples with the sets of received signal samples and rotation elements in such a way as to simultaneously estimate the DC offset (adc) and the channel impulse response (h) by minimising a squared distance function.
According to another aspect of the invention there is provided a method of correcting for a DC offset in a signal received from a first station by a second station via a communication channel in a mobile communication system, the signal comprising digital data and a set of training sequence bits modulated prior to transmission, the method comprising: receiving and sampling the signal to produce a plurality of received signal samples from the training sequence portion of the signal, the received signal samples possibly including a DC offset; manipulating the received signal samples with a set of reference signal samples representing the training sequence bits and a set of rotation elements depending on the modulation applied to the digital data prior to transmission to simultaneously estimate the DC offset (adc) and the channel impulse response (h) by minimising a squared distance function; and correcting the set of received signal samples in the digital data portion of the signal by removing the thus estimated DC offset from the received signal samples.
Preferably the squared distance function is F=|y−X.h−e.adc|2, where y represents the received signal samples, X represents the training sequence samples and e represents the set of rotation elements
According to a further aspect of the invention there is provided a system for simultaneously determining a DC offset and a channel impulse response in a signal received from a first station by a second station via a communication channel in a mobile communication system, the signal comprising digital data and a set of training sequence bits modulated prior to transmission, the method comprising: circuitry for receiving and sampling the signal to produce a plurality of received signal samples from the training sequence portion of the signal, the received signal samples possibly including a DC offset; a memory holding a set of reference signal samples representing the training sequence bits and a set of rotation elements depending on the modulation applied to the digital data prior to transmission; an extended channel impulse response calculation unit for manipulating the received signal samples with the reference signal samples and the set of rotation elements in such a way as to simultaneously estimate the DC offset (adc) and the channel impulse response (h) by minimising a squared distance function; and means for extracting the DC offset from the simultaneous estimate performed by the extended channel impulse response calculation unit.
The squared distance function is preferably F=|y−X.h|e.adc|2, where y represents the received signal samples, X represents the training sequence samples and e represents the set of rotation elements.
A mathematically convenient way of manipulating the samples is to organise the received signal samples and the rotation elements as an extended training sequence matrix with m+1 columns, where m columns contain the reference signal samples and the m+1th column contains the rotation elements.
The precise manipulation of the received signal samples with the extended reference matrix is discussed in more detail in the following. The inventor has found that by using this extended matrix in a mathematical manipulation, which minimises the noise function w, an estimate can be made simultaneously of the channel impulse response and the DC offset, which can then be removed. For a matrix manipulation, an extended channel impulse vector is generated comprising m channel impulse response elements and a further element adc.
The invention is particularly but not exclusively applicable to a TDMA communication system, where each signal comprises a transmission burst.